Digital microbubble array creates ultrasound holograms
Researchers in Germany have developed a dynamic massive spatial ultrasound modulator (SUM) that can focus acoustic signals into holograms for building microparticles.
The team at the Max Planck Institute for Intelligent Systems, and Institut für Mikroelektronik in Stuttgart developed an array that produces microbubbles on a 0.8um CMOS process to emulate the digital mirror devices used in video projector. An array of 10,000 digitally addressable microbubble pixels on the CMOS chip surface within 12 seconds through water electrolysis. Between frames, the SUM surface is mechanically reset, which creates the first high-resolution animation of sequential acoustic images. The SUM can be used to assemble microparticles into complex shapes.
The acoustic impedance mismatch between gas and liquid means a thin layer of air in liquid can effectively stop ultrasound, even when its thickness is less than the acoustic wavelength. This means a microbubble can serve as a local sound blocker. A pattern of microbubbles in the path of an ultrasound wave therefore create a corresponding amplitude pattern on the wavefront of the acoustic field.
Patterning a large number of microbubbles enables the on-demand shaping of an acoustic field’s amplitude distribution.
The SUM device architecture consists of a CMOS chip placed on top of an acoustic transducer. A liquid film of electrolyte is sandwiched between the chip surface and a conveyor film. The CMOS chip surface has 10,000 individually addressable electrodes on a 70 x 70 μm gold pads in a 100 x 100µm grid.
Positioned next to the chip is a copper electrode, which serves as the anode. A switchable DC power supply provides a potential difference between the copper electrode (+5 V) and the 10,000 gold electrode pads of the CMOS chip. Once the DC power is switched to a CMOS pixel, the electrolysis of the surrounding water solution generates hydrogen and oxygen gas, respectively, at the gold and copper electrodes. As we will see below, the current is controlled to define the size of the microbubbles.
The CMOS chip generates microbubbles according to a binary pattern. Each microbubble corresponds to a location of zero ultrasound transmission. After the bubble generation is completed, the transducer is turned on and the acoustic wave, at 10MHz, transmits through the SUM and is locally blocked at the pixels that are covered by a microbubble. The remainder of the wavefront propagates into the upper container and diffracts to form the target sound pressure distribution.
The SUM generates a pattern of microbubbles on the surface of the CMOS chip by the electrolysis of water. The microbubble coverage has to be large enough to ensure that the acoustic wave is blocked at the location of the electrode. As the potential difference between the anode and the cathode is constant (5 V), the microbubble volume depends on the time the current flows, from 0.6 to 2.8 ms.
For each acoustic image, it takes around 12 s to write the microbubble hologram, when each pixel is sequentially addressed. Afterward, the transducer is turned on for 15 s, generating ultrasound waves, which are modulated by the SUM and propagate to form the acoustic image in the target plane. This forces the microparticles to aggregate into the corresponding shape. After each assembly step, the transducer is turned off, and a motorized film mechanically “wipes” the microbubbles off the chip surface.
Under each electrode, a CMOS transmission gate connects the electrode to a vertical wire. Outside the electrode array, additional transmission gate switches collect the column wires into eight global wires, which lead to the chip pads and can be accessed from the outside of the chip. Two shift register chains, respectively, for row and column select, are fed by a digital driving signal to control the transmission gate groups. The chip is driven by a commercial microcontroller board (Arduino Mega 2560), which is loaded with the codes for chip electrodes addressing and electrolysis voltage switching. The thickness between the conveyor film and the chip surface is estimated to be 20 μm. A 2-μL electrolyte droplet is squeezed between the conveyor film and the chip surface under the experimental conditions, whose spread area is measured as 1 cm2.
The chip was produced by a classical 0.8µm channel length CMOS technology. This p-well technology incorporates local oxidation of silicon device isolation, a single polysilicon layer as the gate electrode, and two aluminium layers for interconnects with a total of 15 optical lithography steps. In addition, two lithography steps specialized post-processing was used for the gold electrodes.
The elements of the chip are currently sequentially addressed, which leads to relatively slow update cycle of 12s, but parallel pixel addressing is expected to drastically increase the refresh rate. To meet the requirements of portable biomedical devices, the bubble removal method can be implemented by other means such as forced fluid flow of the electrolyte or on-chip reversal of the electrolysis.
Future work could explore multilevel amplitude or phase control of sound waves exploiting the resonant behaviour of the microbubbles at specifically controlled sizes. These could be used for medical imaging, nondestructive testing, holographic acoustic tweezers and acoustic fabrication.
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